Secondary Ion Mass Spectroscopy (SIMS) is a surface analysis technique that characterizes materials by determining the mass of the secondary ions that are made to leave the material. To achieve maximum mass resolution, only those secondary ions having energies within a relatively narrow range must be allowed to enter the mass analyzer. See the article entitled "New wide angle, high transmission energy analyzer for secondary ion mass spectrometry", by M. W. Siegel and M. J. Vasile, Rev. Sci. Instrum., 52(11), November 1981, pp. 1603-1615.
Several ion energy filters have been designed to accomplish this. In all of the designs, the ions are subjected to electrostatic or magnetostatic fields, combined with trajectory selecting apertures. Filter designs that produce an electrostatic field between two concentric hemispheres are popular. Unfortunately, as the distance between the hemisphere is increased to permit larger elliptical orbits, the performance is compromised by increasingly large fringe fields between the edges of the two hemispheres.
One ion energy filter in the prior art establishes a force field E with spherical symmetry where E.alpha.(1/r.sup.2), like that which would be produced between two concentric spheres by using one hemisphere on an infinite plane with a potential distribution on the plane that follows the relationship V.alpha.(1/r) where r is the radial distance from the center of the plane. See U.S. Pat. No. 4,126,781 issued Nov. 21, 1978 to M. W. Siegel. Because of the boundary condition established on the plane, a second larger hemisphere is not required, and fringe fields are eliminated.
In the Siegel patent as in the above-identified Siegel et al article, a shaped resistive disk is used to establish the potential distribution proportional to 1/r. This resistive disk is made of a ceramic material impregnated with metal particles. Unfortunately, this impregnated ceramic material is porous and hence incompatible with ultrahigh vacuum applications. It has a poor electrical performance attributable to its nonuniform resistivity and the random localized charging of its surface.